Low Temperature Sintered Ceramic Capacitor With High DC Breakdown Voltage, and Method of Manufacture

Our invention relates to solid dielectric capacitors in general and, in particular, to ceramic capacitors such as those of the monolithic, multilayered configuration that are capable of sintering in a reductive or neutral atmosphere at sufficiently low temperatures to permit cosintering with base metal electrodes for the provision of low cost monolithic capacitors, and to a process for the fabrication of such low temperature sintered capacitors. The ceramic capacitors of our invention are particularly notable for their high DC breakdown voltages per unit thickness of the dielectric body.

Our invention may be considered an improvement of Japanese Laid Open Patent Application No. 59-138003. Described and claimed in in this prior patent application are dielectric ceramic compositions consisting essentially of a major proportion of {(Ba.sub.x Ca.sub.y Sr.sub.z)O}.sub.k (Ti.sub.n Zr.sub.1-n)O.sub.2 and a minor proportion of a mixture of lithium oxide (Li.sub.2 O) and at least one metal oxide (MO) selected from among barium oxide (BaO), calcium oxide (CaO) and strontium oxide (SrO). These ceramic compositions are sinterable to maturity in a reductive or neutral atmosphere. Thus, for the fabrication of monolithic, multilayered ceramic capacitors, for example, the prior art ceramic compositions make possible the costintering of dielectric ceramic bodies and film electrodes of nickel or like base metal interleaved therein.

The ceramic capacitors fabricated from these prior art materials have one weakness, however, particularly when they are used on motor vehicles. Capacitors for vehicular use must retain their full performance characteristics in a temperature range as wide as from -50.degree. to +125.degree. C. The direct current (DC) breakdown voltages of the capacitors in accordance with the prior art ceramic compositions are only from 13 to 18 volts for each micron of the thickness of the dielectric body at the maximum, 125.degree. C., of the expected working temperature range. A conventional solution to this low breakdown voltage problem has been to increase the total thickness of the dielectric body. Such greater size capacitors are of cource objectionable because of the larger space requirements.

We have hereby found out how to improve the noted prior art ceramic compositions for the provision of ceramic capacitors having higher DC breakdown voltages per unit thickness of the ceramic body, and how to manufacture such ceramic capacitors of the improved compositions.

According to our invention, stated broadly, a solid dielectric capacitor is provided which comprises a dielectric ceramic body and at least two electrodes in contact therewith. The dielectric ceramic body consists essentially of 100 parts by weight of a major ingredient set forth below and from 0.2 to 10.0 parts by weight of a mixture of boric oxide (B.sub.2 O.sub.3) and silicon dioxide (SiO.sub.2). The major ingredient is expressed by the compositional formula:

where the subscripts x, y, z and k have the following ranges of values:

and where

The relative proportions of the two additives, B.sub.2 O.sub.3 and SiO.sub.2, are also specified as from 25 to 90 mole percent and from 10 to 75 mole percent, respectively.

The ceramic capacitor of our invention, having its dielectric body formulated as set forth above, has a DC breakdown voltage of 25 volts per micron or more at a temperature of 125.degree. C., compared with 13 to 18 volts per micron in accordance with the prior art. Thus, for a given breakdown voltage, the capacitor of our invention can be much smaller in size than that of the prior art.

Our invention also provides a method of fabricating the above ceramic capacitor. There are first prepared the above indicated proportions of the major ingredient and the additives in finely divided form. After being intimately mingled together, the major ingredient and additives are molded into a body of desired shape and size having at least two electrode portions of an electroconductive material. Then the molding with the electrode portions is sinterend in a nonoxidative (i.e. reductive or neutral) atmosphere and is subsequently reheated in an oxidative atmosphere.

We recommend a temperature range of 1000.degree. to 1200.degree. C. for sintering the dielectric body. This temperature range in an nonoxidative atmosphere is sufficiently low to permit the cosintering of nickel or like base metal as electrodes on the dielectric body without the possibility of the flocculation or diffusion of the base metal. Thus, in preferred examples set forth subsequently, monolithic, multilayered ceramic capacitors are produced in accordance with the ceramic composition and method of our invention. The electrode portions are formed on green sheets of the ceramic composition by coating a paste composed principally of a powdered base metal. Several such green sheets with the base metal coatings thereon are stacked up, pressed together, sintered, reheated, and cut into pieces of required size. A number of monolithic capacitors of an improved voltage withstanding capability and other characteristics can thus be fabricated simultanouesly at reduced cost.

The above and other features and advantages of our invention and the manner of realizing them will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims, with reference had to the attached drawings.

FIG. 1 is a sectional representation of an experimental monolithic, multilayered ceramic capacitor embodying the principles of our invention, the capacitor being representative of numerous test capacitors fabricated in the Examples of our invention to be presented subsequently; and

FIG. 2 is a fragmentary section through a plurality of green ceramic sheets, having an electroconductive paste coated thereon in a required electrode pattern, to be processed into monolithic, multilayered capacitors each constructed as in FIG. 1.

We have illustrated in FIG. 1 one of many monolithic ceramic capacitors fabricated in various Examples of our invention by way of a preferable embodiment thereof. Generally designated 10, the representative capacitor is of multilayered configuration, having an alternating arrangement of seven dielectric ceramic layers 12 and six buried film electrodes 14. The dielectric layers 12 constitute in combination a dielectric body 15 of the improved compositions of our invention, to be detailed subsequently, having the film electrodes 14 buried therein. The film electrodes 14 can be fabricated from a low cost base metal such as nickel. Three of the six film electrodes 14 extend in parallel spaced relation to one another from a first side of the dielectric body 15 toward its second side opposite to the first side and terminate short of the second side. The other three film electrodes 14 extend in parallel spaced relation to one another from the second side of the dielectric body and terminate short of its first side. The two groups of film electrodes 14 are in staggered, interdigitating configuration.

A pair of conductive terminations 16 contact respectively the two groups of film electrodes 14. Each termination 16 comprises a baked on zinc layer 18, a plated on copper layer 20, and a plated on solder layer 22.

We fabricated 67 different sets of test capacitors, each constructed as in FIG. 1, some in accordance with the ceramic compositions of our invention and others not, and measured their electrical properties. Table 1 lists the compositions of the dielectric bodies of the test capacitors fabricated. We have generally formulated the major ingredient of the ceramic compositions of our invention as {(Ba.sub.1-x-y Ca.sub.x Sr.sub.y)O}.sub.k (Ti.sub.1-z Zr.sub.z)O.sub.2. Thus, in Table 1, we have given the specific values of (1-x-y), x, y, k, 1-z, and z. The ceramic compositions of our invention further includes a mixture of additives B.sub.2 O.sub.3 and SiO.sub.2. Table 1 specifies the amounts, in parts by weight, of this additive mixture with respect to 100 parts by weight of the major ingredient, and the relative proportions, in mole percent, of the two additives.

In Test No. 1 of Table 1, for instance, the major ingredient of the dielectric bodies of the test capacitors was {(Ba.sub.0.78 Ca.sub.0.10 Sr.sub.0.12)O}.sub.1.01 (Ti.sub.0.86 Zr.sub.0.14)O.sub.2. One hundred parts by weight of this major ingredient was admixed with 1.0 parts by weight of a mixture of 40 mole percent B.sub.2 O.sub.3 and 60 mole percent SiO.sub.2.

For the fabrication of the test capacitors of Test No. 1, we started with the preparation of the major ingredient. We provided the following start materials:

BaCO.sub.3 . . . 574.05 grams (78.8 mole parts)

CaCO.sub.3 . . . 37.47 grams (10.1 mole parts)

SrCO.sub.3 . . . 66.08 grams (12.1 mole parts)

TiO.sub.2 . . . 254.16 grams (86.0 mole parts)

ZrO.sub.2 . . . 68.23 grams (15.0 mole parts)

These start materials had all purities of not less than 99 percent. The above specified weights of the start materials do not include those of the impurities. We charged these start materials into a pot mill together with alumina balls and 2.5 liters of water and mixed them together for 15 hours. Then the mixture was introduced into a stainless steel receptacle and dried by air heated to 150.degree. C. for four hours. Then the dried mixture was crushed into relatively coarse particles, which were subsequently charged into a deep receptacle for firing in air within a tunnel furnace at 1200.degree. C. for two hours. There was thus obtained the major ingredient of the above specified composition in finely divided form.

For the provision of the additive mixture of Test No. 1, we prepared:

B.sub.2 O.sub.3 . . . 43.58 grams (40 mole percent)

SiO.sub.2 . . . 56.42 grams (60 mole percent)

To this mixture we added 300 cubic centimeters of alcohol, and the resulting mixture was stirred for 15 hours in a polyethylene pot with alumina balls. There was thus obtained the desired additive mixture of 40 mole percent B.sub.2 O.sub.3 and 60 mole percent SiO.sub.2.

Then 1000 grams (100 weight parts) of the above prepared major ingredient and 30 grams (three weight parts) of the above prepared additive mixture was mixed together. To this mixture we added 15 percent by weight of an organic binder and 50 percent by weight of water with respect to the total weight of the major ingredient and the additive mixture. The organic binder was an aqueous solution of acrylic ester polymer, glycerine, and condensed phosphate. The mixture of all these was ball milled into a slurry. Then this slurry was defoamed in vacuum. Then the defoamed slurry was introduced into a reverse roll coater thereby to be shaped into a thin, continuous strip on an elongate base strip of polyester film. Then the strip on the base film was dried by heated heated to 100.degree. C. The green (unfired) ceramic strip thus obtained, about 25 microns thick, was subseuently cut into seven "squares" 24, FIG. 2, each sized 10 by 10 centimeters. These green ceramic squares are to become ceramic layers 12, FIG. 1, in the completed test capacitors.

For the provision of the base metal film electrodes 14 on the ceramic layers 12, we prepared 10 grams of nickel in finely divided form, with an average particle size of 1.5 microns, and a solution of 0.9 gram of ethyl cellulose in 9.1 grams of butyl "Carbitol" (trademark for diethylene glycol monobutyl ether). Both were agitated for 10 hours to provide an electroconductive paste. Then this paste was screen printed in rectangular patterns, each sized 2.5 by 7.0 millimeters, on one surface of each of all but one of the seven green ceramic squares 24 which had been prepared as above.

FIG. 2 indicates at 26 the printings of the electroconductive paste on the six green ceramic sequres 24. It will be observed that the printings 26 on the first, third and fifth squares, counted from top to bottom in FIG. 2, are offset from those on the second, fourth and sixth squres approximately one half of their longitudinal dimensions.

After drying the printed paste at 50.degree. C. for 30 minutes, the six green ceramic squares 24 with the printings 26 thereon were stacked up, with their printings directed upwardly. The other one square which had not been printed was placed on top of the stack of six printed squares, as in FIG. 3. Then the stack of seven squares was pressed in their thickness direction under a pressure of approximately 40 tons at 50.degree. C., thereby firmly uniting them together. Then the united squares were cut in a latticed pattern, as indicated by the dot and dash lines in FIG. 2, into 400 pieces.

We employed a furnace capable of atmosphere control for cofiring the above prepared green dielectric bodies having buried therein the electroconductive layers which were to become the film electrodes 14. The bodies were first air heated in the furnace to 600.degree. C. at a rate of 100.degree. C. per hour, thereby driving off the organic binder that had been used for providing the slurry of the powdered major ingredient and additive mixture. Then, with the furnace atmosphere changed from air to a reductive (nonoxidative) one consisting of two percent by volume of molecular hydrogen and 98 percent by volume of molecular nitrogen, the furnace temperature was raised from 600.degree. C. to 1090.degree. C. at a rate of 100.degree. C. per hour. The maximum temperature of 1090.degree. C., at which the ceramic bodies were to be sintered to maturity, was maintained for three hours. Then the furnace temperature was lowered to 600.degree. C. at a rate of 100.degree. C. per hour. Then the furnace temperature was returned to air (oxidative atmosphere), and the temperature of 600.degree. C. was maintained for 30 minutes in that atmosphere for the oxidative heat treatment of the sintered bodies. Then the furnace temperature was allowed to drop to room temperature.

There were thus obtained the dielectric ceramic bodies 15, FIG. 1, cosintered with the film electrodes 14 buried therein. Each dielectric layer 12 had a thickness of 0.02 millimeter. That portion of each film electrode 14 which is opposed to a portion of the neighboring film electrode had an area of approximately five square millimeters (1.93.times.2.60). The total area of the opposed portions of the six film electrodes 14 was approximately 30 square millimeters.

We found that the composition of the sintered ceramic bodies 15 was substantially the same as that before sintering. It is reasoned, therefore, that the ceramic bodies 15 are of composite perovskite structures, with the additive mixture (40 mole percent B.sub.2 O.sub.3 and 60 mole percent SiO.sub.2) uniformly dispersed among the crystal grains of the major ingredient, {(Ba.sub.0.78 Ca.sub.0.10 Sr.sub.0.12)O}.sub.1.01 (Ti.sub.0.86 Zr.sub.0.14)O.sub.2. The (Ba.Ca.Sr)(Ti.Zr)O.sub.3 ceramics, so to say, of our invention contain BaTiO.sub.3, BaZrO.sub.3, CaTiO.sub.3, CaZrO.sub.3, SrTiO.sub.3, and SrZrO.sub.3, and the ingredients of the additive mixture among the crystal grains. In cases where k is greater than 1 in the formula of the major ingredient, excess quantities of BaO, CaO and SrO are contained with respect to the perovskite crystals.

Following the firing of the ceramic bodies 15 we proceeded to the production of the pair of conductive terminations 16 on both sides of each ceramic body. First, for the production of the inmost zinc layers 18, an electroconductive paste of zinc, glass frit and vehicle was coated on both sides of each ceramic body 15. The coatings were dried and air heated at 550.degree. C. for 15 minutes, thereby completing the zinc layers 18 each in direct contact with three of the six film electrodes 14 buried in the ceramic body 15. Then the intermediate copper layers 20 were formed on the zinc layers 18 by electroless plating. The outermost solder layers 22 were formed subsequently by electroplating a lead tin alloy on the copper layers 20.

We have thus completed the fabrication of monolithic, multilayered ceramic test capacitors, each constructed as in FIG. 1, in accordance with the ceramic composition of Test No. 1 of Table 1. As regards the other ceramic compositions of Table 1, designated Tests Nos. 2 through 89, we manufactured similar monolithic, multilayered capacitors through exactly the same procedure as that of Test No. 1 except for the temperature of firing in the reductive (nonoxidative) atmosphere.

Ten random samples of the 400 capacitors of each Test were then tested as to their specific dielectric constant, dielectric loss, resistivity, and DC breakdown voltage. We measured these electrical properties as follows:

1. Specific Dielectric Constant

The capacitance of each test capacitor was first measured at a temperature of 20.degree. C., a frequency of one kilohertz, and an effective voltage of 0.5 volt. Then the specific dielectric constant was computed from the capacitance measurement, the total area (30 square millimeters) of the opposed parts of the film electrodes 14, and the thickness (0.02 millimeter) of each ceramic layer 12.

2. Dielectric Loss

Under the same condiditons as the specific dielectric constant.

3. Resistivity

Resistance between the pair of conductive terminations 16 of each test capacitor was measured after the application of a direct current at 50 volts for one minute at 20.degree. C. Then resistivity was computed from the resistance measurement and the size of each test capacitor.

4. DC Breakdown Voltage

The test capacitors were introduced into a thermostatic oven, in which they were maintained at 125.degree. C. for 30 minutes. Then, at that temperature, a DC voltage was impressed across each test capacitor and was increased at a rate of 100 volts per second, until the test capacitor broke down. The voltage measurement at the breakdown of each test capacitor was divided by the thickness, 0.02 millimeter (20 microns) of each ceramic layer 12.

Table 2 gives the results of the measurements by the above methods, as well as the temperatures of sintering in the reductive atmosphere. It will be seen from this table that the specific dielectric constants of the Test No. 1 capacitors, for instance, averaged 11,510, their dielectric losses 1.0 percent, their resistivities 3.12.times.10.sup.6 megohm-centimeters, and their DC breakdown voltages 32.5.1 volts per micron, that is, 650 volts for the thickness (20 microns) of each ceramic layer 12.

Prior to a study of the results of Table 2 we will determine the acceptable criteria of the four electrical properties in question for the ceramic capacitors provided by our invention. These criteria are:

Specific dielectric constant:

At least 5000.

Dielectric loss:

Not more than 2.5 percent.

Resistivity:

Not less than 1.times.10.sup.4 megohm-centimeters.

DC breakdown voltage:

Not less than 25 volts per micron.

An examination of Table 2 in light of the above established criteria of favorable electrical characteristics will reveal that the capacitors of Tests Nos. 7, 8, 13, 18, 19, 23, 24, 28, 29, 33, 34, 38, 39, 50, 58, 59, 60 and 66 do not meet these criteria. Accordingly these capacitors fall outside the scope of our invention. All the other test capacitors satisfy the criteria and so are in accord with our invention.

Now, let us consider the ceramic compositions of Table 1 and the corresponding capacitor characteristics, as well as the sintering temperatures, of Table 2 in more detail. The ceramic composition of Test No. 8 contained no additive mixture specified by our invention. The resulting dielectric bodies had to be heated to as high as 1350.degree. C. for sintering to maturity. Further the resulting capacitors had a dielectric loss of as high as 4.9 percent and a DC breakdown voltage of as low as 3.1 volts per micron. Take the ceramic compositions of Tests Nos. 2 and 9 for comparison. They include 0.2 parts by weight of the additive mixture with respect to 100 parts by weight of the major ingredient. Even though the sintering temperatures were as low as 1180.degree. and 1170.degree. C., the Tests Nos. 2 and 9 capacitors possess the electrical characteristics coming up to the above criteria. We set, therefore, the lower limit of the possible proportions of the additive mixture at 0.2 part by weight with respect to 100 parts by weight of the major ingredient.

The ceramic composition of Test No. 7 contained as much as 12 parts by weight of the additive mixture. The resulting Test No. 7 capacitors had a dielectric loss of 4.7 percent, which is higher than the desired criterion. However, when the proportion of the additive mixture was reduced to 10 parts by weight, as in Tests Nos. 6 and 12, the resulting capacitors had the desired characteristics. Accordingly, the upper limit of the possible proportions of the additive mixture is set at 10 parts by weight with respect to 100 parts by weight of the major ingredient.

Tests Nos. 13 through 28 are intended primarily to determine the effectcs of the various relative proportions of the additive mixture on the characteristics of the resulting capacitors. As indicated by Tests Nos. 13 and 19, the ceramic compositions containing only 20 mole percent B.sub.2 O.sub.3 were not sintered to maturity at a firing temperature of 1250.degree. C. However, as in Tests Nos. 14, 20 and 25, capacitors of desired characteristics could be obtained when the proportion of B.sub.2 O.sub.3 was increased to 25 mole percent. The minimum possible proportion of B.sub.2 O.sub.3 is therefore 25 mole percent. When the B.sub.2 O.sub.3 proportion was increased to 95 mole percent, as in Tests Nos. 18, 23 and 28, the resulting capacitors had dielectric losses of more than three percent and breakdown voltages of less than 25 volts per micron. Capacitors of desired characteristics could be obtained when the B.sub.2 O.sub.3 proportion was reduced to 90 mole percent as in Tests Nos. 17, 22 and 27. The maximum possible proportion of B.sub.2 O.sub.3 is therefore 90 mole percent. Inevitably, therefore, the acceptable range of proportions of the other additive, SiO.sub.2, is from 10 to 75 mole percent.

Tests Nos. 29 through 38 were intended to ascertain the effects of the various values of k in the formula of the major ingredient on the characteristics of the resulting capacitors. The value of k was set at 0.99 in Tests Nos. 29 and 34. The dielectric losses, resistivities, and breakdown voltages of the resulting capcitors were much worse than the above criteria. However, when the value of k was increased to 1.05 as in Tests Nos. 33 and 38, the resulting dielectric bodies were not coherently bonded on firing. As demonstrated by Tests Nos. 30, 31, 32, 35, 36 and 37, the capacitors of the desired characteristics could be obtained when the values of k were in the range of 1.00 through 1.04.

The values of x, y, z and k in the general formular of the major ingredient were variously determined in Tests Nos. 39 through 67 to test the various proportions of barium (Ba), calcium (Ca), strontium (Sr), titanium (Ti) and zirconium (Zr). Both x and y were zero, with Ca and Sr deleted from the major ingredient, in Test No. 39. The resulting dielectric bodies were difficult of oxidation by the heat treatment at 600.degree. C., and the dielectric losses, resistivities, and breakdown voltages of the resulting capacitors did not come up to the desired criteria. Contrastively, the desired characteristics were obtained by the Test No. 40 capacitors in which x was 0.02. However, when the value x was made as much as 0.28 as in Test No. 50, the specific dielectric constant became lower than the criterion. The desired electrical characteristics could all be obtained when x was 0.27 as in Test No. 49. We conclude from these findings that the values of x should be in the range of 0.02 to 0.27.

The value of y in the formula of the major ingredient was set at 0.39 in Test No. 59. The specific dielectric constant of the resulting capacitors was below the criterion. The capacitor characteristics were all satisfactory when the value of y was set at 0.37 and 0.001 as in Tests Nos. 44 and 45. Consequently, the value of y should be not more than 0.37. Additional experiment has proved that even if the value of y is less than 0.001, capacitors of better characteristics are obtainable than if y is zero.

The value of (1-x-y) in the formula of the major ingredient was set at 0.60 in Test No. 57, and the resulting capacitors possessed the desired characteristics. However, when the value was lowered to 0.58 as in Test No. 58, the specific dielectric constant of the resulting capacitors become lower than the criterion. The lowermost possible value of (1-x-y) is therefore 0.60. The highest possible value of (1-x-y) must be less than 0.98 since, as we have specified above, x is at least 0.02 and y is at least greater than zero.

Test No. 60 indicates that the specific dielectric constant, dielectric loss and DC breakdown voltage of the capacitors are poor if z is zero. Test No. 66 proves, on the other hand, that the specific dielectric constant also becomes poor if the value of z is made as high as 0.28. The desired electrical characteristics could all be obtained if the value of z was in the range of 0.01 to 0.26 as in Tests Nos. 61 through 65. Additional experiment has proved that even if the value of z is less than 0.01, capacitors of better characteristics are obtainable than if z is zero. Thus the value of z must be greater than zero and not more than 0.26. Consequently, (1-z) is at least 0.74 and less than one.

By way of comparison of our invention with what we believe to be the closest prior art, we manufactured, as Tests Nos. 68 through 75, capacitors of exactly the same size and construction as those set forth in connection with Test No. 1, only with the ceramic compositions formulated in accordance with the aforesaid Japanese Laid Open Patent 15 Application No. 59-138003. These test capacitors were fabricated by the same method as in Tests Nos. 1 through 67. Table 3 lists the prior art ceramic compositions of Tests Nos. 68 through 75, and Table 4 the sintering temperatures of the ceramic bodies and the four electrical properties in question of the resulting capacitors. The electrical properties were also measured by the same methods as with Test Nos. 1 through 67.

As will be seen from Table 4, the DC breakdown voltages of the prior art capacitors of Tests Nos. 68 through 75 all fall short of the criterion of 25 volts per micron. It is therefore apparent that our invention has succeeded in marked improvement in the breakdown voltages of the capacitors of the class under consideration.

Although we have disclosed our invention in terms of specific Examples thereof, we understand that our invention is not to be limited by the exact detals of such disclosure but is susceptible to a variety of modifications within the usual knowledge of the ceramics or chemicals specialists without departing from the scope of our invention. The following, then, is a brief list of such possible modifications:

1. The ceramic compositions of our invention might include various additives not disclosed herein. An example is a mineralizer such as manganese dioxide. Used in a proportion (preferably from 0.05 to 0.10 percent by weight) not adversely affecting the desired properties of the resulting capacitors, such a mineralizer would improve the sinterability of the dielectric bodies.

2. The start materials of the major ingredient might be substances other than those employed in the Examples herein, such as BaO, SrO, CaO and other oxides or hydroxides. The start materials of the additive mixture might also be other oxides, hydroxides, etc.

3. The temperature of reheating in an oxidative atmosphere might be variously determined in a range (from 500.degree. to 1000.degree. C. for the best results) less than the sintering temperature (from 1000.degree. to 1200.degree. C.), in consideration of such factors as the particular base metal electrode material in use and the degree of oxidation required.

4. The temperature of costintering in a nonoxidative atmosphere might also be changed in consideration of the particular electrode material in use.

5. The dielectric bodies with the buried electrodes could be sintered in a neutral, instead of reductive, atmosphere.

6. The ceramic compositions of our invention might be used as the dielectric bodies of capacitors other than those of the monolithic, multilayered configuration disclosed herein. In its simplest form, therefore, the capacitor fabricated in accordance with our invention would comprise a dielectric body and two electrodes in contact therewith.